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RE: 10 mm coils for wireless receiver

Akio Tanaka
Prodigy 10 points
Other Parts Discussed in Thread: BQ500212AHi, Dick

I am experimenting with Rx 10mm and Tx 15mm coil (not Qi-compliant) using the TIDA-00334 reference design. Tx coil has a large temperature increase (at f~110kHz) because of the small coil, so I am trying to set the current limit of 0.4A using the I_SENSE of Tx side. In order to use the current limit, it looks like there is a need to activate the PMOD. When I set a PMOD resistor of Tx and pull up the FOD of Rx to RECT, it looks like I_SENSE of Tx has a threshold of 2V. By changing the current detection resistor from 20m-ohm to 100m-ohm, it appears to be the current limit is set at 0.4A. I would appreciate it very much if you let me know the information around this. Thank you.

Regards,

Akio
  • 0
    TI__Guru 65846 points
    Hello Akio Tanaka
    The Transmitter coil will have a high circulating current flowing though it under light or no load condition. Current will only go up with load. DC resistance of the TX coil should be kept low to keep thermal rise low. Standard TIDA-00334 TX coil (30mm) the DCR is 150m ohms and has shown good thermal performance. I recommend keeping the TX coil resistance in this range.
    bq500212A (TIDA-00334) does not have an input current limit function that will reduce the output power. Changing the 20m ohm sense resistor will change the current sense measurement but only for FOD and PMOD fault protection.
    PMOD only applies to WPC 1.0 version of TI receivers. Later receivers (WPC 1.1) will use FOD. Trying to use this function as a power limit is very difficult and could easily result in the unit going into a fault shut down.

    I recommend taking a look a the Wurth 760308101104 TX coil used on TIDA-00415 this a 20mm coil that will give good thermal performance and small size. Other TX coils are available from TDK.

    A 10mm coil is difficult to work with. Keep the coil to coil distance low.